This invention relates to a gas sensing element using a semiconductor made of sintered metal oxide, for detecting the partial pressure of oxygen gas in an gas atmosphere from a variation in the electrical resistance of the semiconductor.
The conventional gas sensing element includes a semiconductor made of a sintered oxide of a metal such as titanium dioxide cobalt, tin or chromium, and a pair of electrodes embedded in the semiconductor, as lead wires across the electrical resistance of the semiconductor. This gas sensing element has been extensively employed as a gas component detecting device to control the exhaust gas of a combustion machine, such as an internal combustion engine, to decrease emission.
To form a gas sensing element of this type, a pair of electrodes are embedded in a mass of metal oxide powder, which is the raw material of the semiconductor. The electrodes and the mass of metal oxide powder are molded in a die under pressure and are then subjected to sintering at a temperature of 1200.degree. C. or higher. Therefore, even if sintering is carried out in an inert atmosphere, the electrodes are unavoidably oxidized. Accordingly, the electrodes are made of expensive platinum or a platinum alloy such as platinum-rhodium (hereinafter referred to as "platinum metal"), to prevent oxidation. On the other hand, the semiconductor made of the sintered oxide (hereinafter referred to as "an oxide semiconductor") is made porous to increase the gas sensing ability. Therefore, CO and HC in the exhaust gas enter the porous semiconductor and come in contact with the platinum metal electrodes, and carbon is deposited by their catalytic reaction. This carbon accumulates on the electrodes, causes stress on the oxide semiconductor, expanding the latter, and occasionally destructive failure are occurred. In this point, the conventional gas sensing element is disadvantageous.